348 



CiiiToxiD-i:.- 



-JIOLLUSCA.- 



-ClIITOXID.E. 



to a consideraMe size, as the Chiton srjvamosus, &c. (see 

 fig. antiexcd); tliese are eagerl 3' devoured by the iiegioes, 

 who call them beef. The thiuk fleshy foot is cut away 

 from the living animal, and swallowed raw, while the 

 viscera are rejected. The Chitons deposit their eggs, 

 closely united in clusters, on stones, each egg being 

 inclosed in a thick, vesicular, folded envelope. The 

 young, when first hatched, have no shell ; that covering 

 not being developed until some time after. The back 

 of the newly hatched creature, however, is marked with 

 .seven cross furrows, and between them, the first rudi- 

 ments of the shell make their appearance in the form 

 of close granulations. As the animal increases in size, 

 the furrows become more distinctly separated, and the 

 first shell is then seen in the form of seven narrow bands 

 with irregular margins. In a family containing upwards 

 if two hundred species, it may easily be imagined the 

 varieties in form, &c., must be considerable. The 

 Chitons accordingly have been divided into no fewer 

 than twenty-three different genera, and in the most 

 recent arrangement of these. Dr. Gray has selected, as 

 characters, the forms of the plates of insertion of the 

 various valves. We must refer to his " Systematic 

 Arrangement" for the details of the various genera. 

 We will content ourselves in this place with merely 

 stating, that they may all be arranged in two groups or 

 snb-families depending upon the structure of the mantle. 

 The true Chitons, the Chitonince of Adams, have the 

 mantle simple, and without pores on it furnished with 

 tuffs of bristles or spines. The CrijptopJacincs of 

 Adams, the second group, have the mantle furnished 

 with pores which are beset with subulate, flexible 

 bristles, or with tufts of bristle-like calcareous spines. 



Group I.— Ciiitonin^. 



This group is by far the most numerous, and the 

 eilge of the mantle is variously covered by scales or 

 calcareous thick bristles, like spines ; or it is smooth 

 and naked. In the genus Chiton, the mantle margin 

 is covered with regularly disposed, imbricate, smooth, 

 polished scales. — (See fig. annexed of Chiton squa- 

 mosus). 



In the genus ACANTHOrLEURA, the mantle margin is 

 covered with uniform shelly spines or elongated scales; 

 and in the genus TONICIA, the mantle margin is naked, 



smooth, and covered with a liard shining skin beneath. 

 In all these the exposed part of the valves, or the part 

 not covered by the mantle of the animal, is broad and 

 band-like ; but in the curious genus CnjxAochiton, the 



Fig. 227. 



Chiton sqnamosus. 



valves are entirely covered by the mantle, and this is 

 set all over with numerous close, uniform tufts of cal- 

 careous spicula, which in a dry state resemble at first 

 sight particles of saw-dust. 



Group II. — Cryptoplacin^. 



This group contains fewer species, and some of ihese 

 present a good deal of similarity to annulose animals, 

 from the smallness of the exposed portion of the shell, 

 and the tufts of spines along the edge of the mantle. 

 In Plaxiphora the pores of the mantle are furnished 

 with subulate, flexible bristles. In Aainthoclietes the 

 pores are furnished with tufts of bristle-like calcareous 

 spines, which are generally large and often iridescent. 

 The exposed part of the valves is moderately broad. 

 In Chitonclius, the mantle is covered with short, 

 crowded, calcareous spines, and the pores on the sides 

 are small, with similar spines; but the exposed portion 

 of the valves is rather small, and the body of the animal 

 is elongate, subcylindrical, and the back is convex. It 

 is this pectdiar form of the animal that causes them to 

 resemble so much some of the Annelids. 



Sub-class II.— HETEROBRANCIIIATA. 



In this sub-class of Molluscs, the gills are not, as in 

 tlie former sub-class (see p. 186), inclosed in a special 

 cavity, but are more or less exposed on the back, or 

 at the sides towards the hind part of the body. Their 

 respiration, however, is aquatic as in the ctenobran- 

 cliiate orders; and the animals are hermaphrodite, the 

 male and female organs being found in the same in- 

 diviilual. 



The number of species contained in this sub-class is 



very considerable, though certainly inferior to those of 

 the preceding; and many of the genera have no external 

 covering or shell in the adult state. The young or larva', 

 however, are in all of them provided with a spiral shell, 

 the aperture of which is closed by an operculum. Their 

 head is furnished with two lobes or fins, fringed at the 

 margin with vibratile cilia, which serve as organs of 

 locomotion, but which disappear as they reach the 

 mature state. 



